Credit: NASA

Only with the advent of space oceanography have spiral structures in the sea become recognized phenomena. They have proved rewarding, if difficult, subjects for research.

Walter Munk and colleagues have analysed 400 photographs from space that show these attractive patterns on the ocean surface (Proc. R. Soc. Lond. A 456, 1217–1280; 2000). They restricted their investigation to small spirals, 10–25 km in diameter, that last for only about a day, excluding features such as the rings in the Gulf Stream that can be up to 300 km across and last for years.

It turns out that most spirals rotate in the same direction as storm cyclones: anticlockwise and clockwise in the Northern and Southern Hemispheres, respectively. Munk et al. find that all potential mechanisms favour the same direction of motion: because of the Earth's rotation, cyclonic spirals form more easily and are more stable. Moreover, cyclonic rotation is more likely to be visible, because streaks on the sea surface that can be twisted into spirals are more likely to occur where cyclones are created.

For much the same reasons, the Mediterranean in autumn is particularly rich in visible spirals: the seasonal, strong winds, followed by calm conditions, are ideal generators of streaks. The picture here shows an example of fully developed spirals off Crete in October 1984.

Similar eddies have been explained as stemming from instabilities at the interface between two water masses of different densities. But that cannot apply to many of the images studied by Munk et al. Rather, they think that the driving force is often turbulence generated at the boundary of two water masses moving relative to each other.

As the authors remark, however, there is certainly more than one way to make an ocean spiral. Which may explain why they are such common features of the seascape.